Building Stones of Great Edifices: Stonehenge

This will be the first of what I hope to be periodic postings about the building stones of famous edifices, such as the Great Pyramids, Machu Pichu, and the Great Wall of China.  Suggestions would be welcome. 

Some of the world’s oldest building stones made it into a fine story in this month’s issue of Earth.  Written by English geologist Brian S. Johns and Lionel E. Jackson, Jr., a Quaternary geologist from British Columbia, the article offers a thought provoking and believable account of the travels of the enigmatic rocks used at Stonehenge.  In doing so, they answer one of the great questions posed by anyone who has visited the Salisbury Plain, “How the hell did they do it?” 

From wikipedia

As Johns and Jackson note, this question has led to some dubious answers.  The Celts, Vikings, Phoenicians, Druids, and Romans have all had their day in the spotlight, though the building of Stonehenge, around 4,500 years ago, predates each of these peoples.  Nor were space aliens involved, though that would be a very handy way to explain the many mysteries.  Johns and Jackson don’t provide an answer to who but instead focus on the origin of the stones.  

Two primary types of building stone make up the majority of the monoliths.  The outer ring consists of a 60-million-year old rock known as sarsen sandstone.  (Sarsen is a regional term applied to the boulders found scattered across south central England.)  About 50 sarsen stones occur, either as vertical slabs or horizontal lintels, and comprise Stonehenge’s most famous features, the Pi-shaped trilithons.  The largest sarsens weigh an estimated 40 tons, or about equal in weight to 48 Smartcars.  

The smaller and less abundant bluestones raise more questions.  Made primarily of diabase, but also rhyolite and additional volcanic material, the stones’s origin has been traced to the Preseli Hills of western Wales, more than 125 miles to the west.  At least eight different rock types associated with Stonehenge have been found across seven miles of hills in this region of Wales.  Early researcher Herbert Thomas, who promoted the “human transport” theory for the origin of the bluestones, hypothesized that Stonehenge’s builders sought out these stones because of their magical and medicinal properties. 

Source area for bluestones: From Brian Johns’ web site

Johns and Jackson favor a glacial transport model for the bluestones.  They describe how converging ice sheets from Ireland and Wales funneled erratics from Wales in a “trail leading straight to Stonehenge.”  As evidence of such a conveyor belt-like feat, they report on glaciers in Canada, which ferried erratics in a narrow band over 350 miles south.  In England, this phenomenon provided Neolithic Britons with a ready source of stone for their great structure, no matter why they built it.  And that is the great mystery that not even geology can solve.

Deccan Traps: A Different Take

At the recent 2008 AGU meeting in San Francisco, a long simmering controversy reared its head again.  Paleontologists Gerta Keller and Sunil Bajpai and geophysicist Vincent Courtillot presented evidence that India’s Deccan Traps, and not the Chicxlub meteor in Mexico, created the unhealthful conditions that led to the extinction of the dinosaurs.  Their data is thought provoking but what attracted my attention was Dr. Keller’s comment that much of their data came from quarries in the basalt.  

Erupted 65 million years ago, the Deccan Traps covers an area as large as Texas.  Geologists have estimated the volume at 1.2 million cubic kilometers, with nearly half lost to erosion.  The depth of the layers is more than 3,500 meters thick.  At AGU, Keller observed that the viscous basalt spewed forth in as little as 10,000 years. 

Basalt Quarry: Photo from Gerta Keller

The quarries that interest Keller and her colleagues are found in flows that oozed 800 kilometers across India to Rajahmundry, on India’s east coast.  They are the longest lava flows on Earth.  Dozens of quarries pockmark the Rajahmundry traps of the Deccan plateau.  (Trap refers to any dark colored igneous rock though it is most commonly associated with basalt; trap comes from the Swiss word for step.)  According to Dr. Keller, families and extended clans work most of the quarry sites by hand, using hammers and explosives.  Men break up the stone and women carry it out on their heads.  And in some cases, trucks transport rock to people’s homes and dump the material in their backyards, where they work on it. 

Worker at basalt quarry: Photo from Gerta Keller

Because the Rajahmundry basalt resists weathering, much of it goes for roads and to make train beds.  It is shipped to Europe and perhaps to the United States.  Basalt from other flows in India have been used as a building stone, though Dr. Keller did not know if the Rajahmundry stone had gone into buildings. 

The quarries have played an important role in Dr. Keller’s research because they expose the rocks that she has wanted to study.  In the quarries is evidence for shallow marine deposition, where marine microfossils were preserved.  These fossils have been essential to narrowing the date of Deccan Trap deposition and allowing Dr. Keller to further elaborate on her thesis. I am not qualified to say whether her theory is right or wrong, but the work raises some interesting questions and if Dr. Keller is right, you may want to give a second thought to the road you travel on.  It may contain evidence for the dinosaur’s demise.